T277 Fresh Perspectives on Critical Transitions in Earth History: Insights from Novel Tools or New Successions

Wednesday, 4 November 2015
8:00 AM - 12:00 PM
Room 319/320

RECONSTRUCTION OF LOCAL AND GLOBAL MARINE REDOX CONDITIONS DURING DEPOSITION OF LATE ORDOVICIAN AND EARLY SILURIAN ORGANIC-RICH MUDROCKS IN THE SILJAN RING DISTRICT, CENTRAL SWEDEN

Paper number: T277-13
Program: Technical Sessions
Time: 10:50 AM – 11:05 AM

Authors

Abstract

The Siljan ring district in central Sweden was created by a bolide impact at 377±2 Ma that triggered oil generation from organic-rich mudrocks (ORM) of the Late Ordovician (Katian) Fjäcka Shale and/or the Early Silurian (Rhuddanian-Telychian) Kallholn Formation. New drill cores obtained by Swedish private company IGRENE AB in 2011 provide an opportunity to significantly improve constraints on the global ocean redox conditions before and after the Late Ordovician Hirnantian glaciation using the U and Mo isotope paleoredox proxies. Here, we analyzed δ238U (relative to standard CRM145 = 0‰) and δ98Mo (relative to standard NIST SRM 3134 = +0.25‰) of 26 ORM samples from the Fjäcka Shale, Kallholn Formation, and latter deposited Nederberga Formation. The extent of Re, Mo, and U enrichment, Re/Mo and U/Mo ratios, and Fe speciation indicate euxinic and oxygenated bottom water conditions during deposition of the Fjäcka Shale and Nederberga Formation, respectively. The same proxies suggest that the Kallholn Formation was deposited under transiently euxinic conditions with the chemocline situated near the sediment-water interface.

The most euxinic shales provide the most relevant estimates of global redox conditions. As expected, the euxinic Fjäcka Shale yields the highest δ98Mo (~1.3‰) and δ238U (~0.1‰) of the studied units. High Mo/TOC ratios (>30 ppm/wt%) of the Fjäcka Shale indicate weak basin restriction and large amounts of Mo in the euxinic bottom waters, which could lead to Mo isotope fractionations between seawater and sediments due to incomplete formation/removal of tetrathiomolybdate. This interpretation is further supported by high U isotope composition in the Fjäcka Shale, which is only slightly lower than the modeled value of 0.2‰ for modern open ocean euxinic sediments. Expanded ocean anoxia should lead to deposition of ORMs with low δ238U (<0‰) as observed during the Cenomanian-Turonian OAE2. Hence, the relatively high δ238U coupled with high Mo, Re, and U enrichments and Mo/TOC ratios in the Fjäcka Shale suggest a more oxygenated ocean prior to the Hirnantian glaciation than previously thought, though the extent of oxygenation was less than today.